The invention relates to the field of telecommunications. It relates more particularly to a device for emitting and/or receiving electromagnetic waves comprising a lens bringing a plurality of directions defined by the radiation space of the lens into correspondence with a plurality of focusing points defining a focusing surface of the lens, the lens comprising a shaped volume of dielectric material. The invention also relates to a telecommunications terminal.
It is known practice, from French patent applications 98/05111 and 98/05112 filed on Apr. 23, 1998 in the name of the Applicant, to use a lens of the Luneberg type in satellite receiving devices, especially for tracking moving satellites.
In theory, the lens must consist of a given number of dielectric layers which is high enough to approach the ideal model of a refractive index varying as a function of the radius, which is characteristic of a Luneberg lens. The refractive index n of a layer and its corresponding dielectric constant xcex5 (or relative permittivity) are thus linked by the equation: n=xcex5xc2xd. However, the increase in the number of layers is limited in practice by severe tolerances which are incompatible with a mass production manufacturing method. For small lenses, typically of a diameter less than 40 cm for transmissions in the Ku band, a solution to this problem is to opt for a lens with a single layer of dielectric material.
In order to reduce the size of the lens while maintaining effective focusing, it is thus necessary to increase the permittivity of the material, the consequence of which is to disadvantageously increase the weight of the lens. A compromise between the focal length of the lens and its weight is therefore necessary. These restrictions of size and weight require the dielectric material to have a well-defined range of permittivity. For example, for a spherical lens of diameter close to 40 cm and of maximum weight approximately equal to 15 kg, the required permittivity is typically between 1.8 and 2.5. On the other hand, the dielectric material chosen must have low dielectric losses (typically a loss angle of less than 0.001 in the Ku band, for example).
It is known from the prior art to use as dielectric material, a mixture comprising expanded polystyrene filled with, for example, high-density particles or ceramic or metallic particles in order to increase the permittivity of the material, changing it to the desired permittivity range. It is also known practice to use, for the same purpose, a mixture of particles of various components (plastic, ceramic or metal, for example) held together by a binder so as to form a composite dielectric material.
However, these types of mixtures do not allow complete homogeneity of the particles to be achieved in the mixture, especially in a large volume, and therefore they do not guarantee homogeneous permittivity in the volume of the lens. Moreover, the mixture obtained is expensive.
The object of the invention is to remedy these drawbacks.
For this purpose, a subject of the invention is a device for emitting and/or receiving electromagnetic waves comprising a lens bringing a plurality of directions defined by the radiation space of the lens into correspondence with a plurality of focusing points defining a focusing surface of the lens, the lens comprising a shaped volume of dielectric material, characterized in that the dielectric material comprises a granular agglomerate defined by a homogeneous or quasi-homogeneous distribution of granules of the same type and of small size with respect to the wavelength of the electromagnetic waves to be received and/or emitted by the said device, the said granules being held under pressure in the said volume by holding means shaped according to the said volume.
Thus, the device according to the invention may be produced at low cost according to a manufacturing method which is compatible with mass production. Since the granules are uniformly distributed in the said volume, homogeneous permittivity is ensured throughout the volume.
According to one embodiment, the said holding means are also designed so that the transition of the electromagnetic waves between the radiation space and the said volume is optimally matched. In this way, reflection losses on the surface of the said holding means are minimized.
Advantageously, in order to pass from the radiation space to the volume of granules, the holding means have a permittivity equal to the square root of the permittivity of the composition of the contents of the volume.
According to one embodiment, the holding means have a thickness of a multiple of a quarter wavelength. These holding means thus act as a matching layer.
According to another embodiment, the thickness of the said holding means is either negligible with respect to the wavelength of the electromagnetic waves to be received and/or emitted, or equal to a multiple of the half wavelength of the said waves to be received or emitted, such that the said holding means are electromagnetically transparent with respect to the said waves.
Advantageously, these holding means are made from a material which is solid enough to provide protection against external attack. For example, the holding means comprise a plastic called acrylonitrile-butadiene-styrene (ABS) covering the said volume. These holding means thus act as a radome.
According to one embodiment, the diameter of the granules is less than one tenth of the wavelength of the electromagnetic waves to be received and/or emitted by the said device.
According to one embodiment, the volume consists of a mixture of granules and air which acts as an artificial dielectric material of equivalent permittivity xcex5equ defined according to the equation:
xcex5equ=((1+2F)xcex5r0+2(1xe2x88x92F))/((1xe2x88x92F)xcex5r0+2+F), 
where F is the ratio of the volume effectively occupied by the granules to the total volume of the said volume (7), and xcex5r0 is the intrinsic permittivity of the granule.
According to one embodiment, the granules are made of a plastic. In this way, the cost price of the device is low.
For example, the granules are made of polystyrene.
In order to increase the permittivity of the dielectric material, each granule is filled with titanium oxide with, for example, a content of 30% by mass.
According to one embodiment, the said holding means comprise a shell a volume of revolution to enable tracking of targets over a solid angle of 360xc2x0 within the context of satellite tracking, for example. This shell is, for example, spherical, hemispherical or cylindrical.
A subject of the invention is also a telecommunications terminal comprising a device for emitting and/or receiving electromagnetic waves comprising a lens for focusing signals received from one direction at a point, the radiation space of the lens determining a set of directions defining a focusing surface, characterized in that the said emitting and/or receiving device is a device according to the invention.